Cellulose, a fibrous, tough, water-insoluble substance is found in the cell walls of plants, particularly, in stalks, stems, trunks and all the woody portions of plant tissues. Cellulose constitutes much of the mass of wood, and cotton is almost pure cellulose. Because cellulose is a linear, unbranched homopolysaccharide of 10,000 to 15,000 D-glucose units, it resembles amylose and the main chains of glycogen. But there is a very important difference, in cellulose, the glucose residues have the beta configuration, whereas in amylose, amylopectin and glycogen, the glucose is in the alpha configuration. The glucose residues in cellulose are linked by (beta 1→4) glycosidic bonds. This difference gives cellulose and amylose very different 3-dimensional structures and physical properties.
Cellulose cannot be used by most animals as a source of stored fuel, because the (beta 1→4) linkages of cellulose are not hydrolyzed by alpha-amylases. Termites readily digest cellulose but only because their intestinal tract harbors a symbiotic microorganism, trichonympha, which secretes cellulase, an enzyme that hydrolyses (beta 1→4) linkages between glucose units. The only vertebrates able to use cellulose as food are cattle and other ruminant animals (sheep, goats, camels and giraffes). The extra stomachs “rumens” of those animals teem with bacteria and protists that secrete cellulase.
The enzymatic hydrolysis of cellulose is considered to require the action of both endoglucanases (1,4-beta-D-glucan glucanohydrolase) and exoglucanases (1,4-beta-D-glucan cellobichydrolase). A synergistic interaction of these enzymes is necessary for the complete hydrolysis of crystalline cellulose. (Caughlin, M. P., Genet. Eng. Rev., 3:39-109 (1985)). For the complete degradation of cellulose (cellulose to glucose), β-glucosidase might be required if the “exo” enzyme does not release glucose. 1,4-β-D-glucan glucohydrolase is another type of “exo” cellulase.
Thermophilic bacteria have received considerable attention as sources of highly active and thermostable cellulolytic and xylanolytic enzymes (Bronneomeier, K. and Staudenbauer, W. L., D. R. Woods (Ed.), The Clostridia and Biotechnology, Butterworth Publishers, Stoneham, Mass. (1993). Recently, the most extremely thermophilic organotrophic eubacteria presently known have been isolated and characterized. These bacteria, which belong to the genus thermotoga, are fermentative microorganisms metabolizing a variety of carbohydrates (Huber, R. and Stetter, K. O., in Ballows, et al., (Ed.), The Procaryotes, 2nd Ed., Springer-Verlaz, New York, pgs. 3809-3819 (1992)).
In Huber et al., 1986, Arch. Microbiol. 144:324-333, the isolation of the bacterium Thermotoga maritima is described. T. maritima is a eubacterium that is strictly anaerobic, rod-shaped, fermentative, hyperthermophilic, and grows between 55° C. and 90° C., with an optimum growth temperature of about 80° C. This eubacterium has been isolated from geothermally heated sea floors in Italy and the Azores. T. maritima cells have a sheath-like structure and monotrichous flagellation. T. maritima is classified in the eubacterium kingdom by virtue of having murein and fatty acid-containing lipids, diphtheria-toxin-resistant elongation factor 2, an RNA polymerase subunit pattern, and sensitivity to antibiotics.
Since, to date, most organisms identified from the archaeal domain are thermophiles or hyperthermophiles, archaeal bacteria are also considered a fertile source of thermophilic enzymes.